Re: NPTL mutex and the scheduling priority

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jun 14, 2006 at 03:19:40PM +0200, S?bastien Dugu? wrote:
> > FUTEX_REQUEUE is used by pthread_cond_signal to requeue the __data.__futex
> > onto __data.__lock.
> 
>   You meant FUTEX_WAKE_OP, I guess. I could not find any place still 
> using FUTEX_REQUEUE in glibc 2.4.

glibc 2.4 uses FUTEX_CMP_REQUEUE, true, but both FUTEX_REQUEUE and
FUTEX_CMP_REQUEUE should behave the same in this regard (after all, they are
implemented using the same futex_requeue routine in the kernel).
FUTEX_CMP_REQUEUE is used in pthread_cond_broadcast, FUTEX_WAKE_OP is used in
pthread_cond_signal.  E.g. nptl/sysdeps/pthread/pthread_cond_broadcast.c:
...
      /* Wake everybody.  */
      pthread_mutex_t *mut = (pthread_mutex_t *) cond->__data.__mutex;
      /* lll_futex_requeue returns 0 for success and non-zero
         for errors.  */
      if (__builtin_expect (lll_futex_requeue (&cond->__data.__futex, 1,
                                               INT_MAX, &mut->__data.__lock,
                                               futex_val), 0))
        {
          /* The requeue functionality is not available.  */
        wake_all:
          lll_futex_wake (&cond->__data.__futex, INT_MAX);
        }
and nptl/sysdeps/pthread/pthread_cond_signal.c:
...
      /* Wake one.  */
      if (! __builtin_expect (lll_futex_wake_unlock (&cond->__data.__futex, 1,
                                                     1, &cond->__data.__lock),
                                                     0))
        return 0;

      lll_futex_wake (&cond->__data.__futex, 1);
    }

  /* We are done.  */
  lll_mutex_unlock (cond->__data.__lock);

	Jakub